First, an uplink power control method of the related art will be described. A main purpose of power control is to compensate for path loss and channel fading so as to guarantee a Signal-to-Noise Ratio (SNR) of a received signal and to improve system throughput by adequate link adaptation. In addition, power control can reduce inter-cell interference. In a 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) system, uplink power control is based on open loop power control involving closed-loop correction. The open loop power control is performed by a terminal and the closed-loop correction is performed by a base station.
FIG. 1 shows basic concept of uplink power control in an LTE system.
As shown in FIG. 1, a terminal (User Equipment (UE)) may measure path loss using an open loop scheme and a base station may control uplink power by a closed-loop correction factor Δ. That is, a transmission power value is determined using the path loss measured by the terminal and a power control value received from the base station.
The power of a Physical Uplink Shared Channel (PUSCH) may be determined as expressed by Equation 1.PPUSCH(i)=min{PMAX,10 log10(M(i))+Po(j)+α(i)·PL+ΔTF(i)+f(i)}[dBm]  Equation 1where, i denotes a time index and PMAX denotes allowed maximum power, which is determined by the terminal. M(i) is determined according to allocated resource blocks, has a value of from 1 to 110, and is updated every subframe. f(i) is varied according to terminal and is determined by the base station.
α(i)PL is used to compensate for path loss, wherein PL is downlink path loss measured by the terminal and α(i) is a scaling value. α(i) is equal to or less than 1. If α(i) is equal to 1, path loss is completely compensated for and, if α(i) is less than 1, path loss is partially compensated for.
P0(j) is calculated by Equation 2.PO=PO—CELL—SPECIFIC+PO—UE—SPECIFIC  Equation 2
The power of a Physical Uplink Control Channel (PUCCH) is expressed by Equation 3.PPUCCH(i)=min{PMAX,PO—PUCCH+PL+h(nCQI,nHARQ)+ΔF—PUCCH(F)+g(i)}[dBm]  Equation 3where, ΔF—PUCCH(F) is received from an upper layer and is determined according to the format of the PUCCH.
h(n) denotes a value determined according to the format of the PUCCH, nCQI denotes the number of bits for channel quality information, nHARQ denotes the number of bits for HARQ.
P0—PUCCH is a sum of P0—NOMINAL—PUCCH which is a cell-specific variable and P0—UE—PUCCH which is a UE-specific variable.
Power control of a sounding reference signal is expressed by Equation 4.PSRS(i)=min{PMAX,PSRS—OFFSET+10 log10(MSRS)+PO—PUSCH(j)+α·PL+f(i)}  Equation 4where, MSRS denotes a bandwidth of SRS transmission of a subframe i.
FIG. 2 is a diagram showing an uplink transmission structure of an LTE system.
As shown in FIG. 2, a PUSCH transmits a value obtained by multiplying a signal to be transmitted by a power value determined by Equation 1 immediately before being transmitted through a physical antenna, and a PUCCH transmits a value obtained by multiplying a signal to be transmitted by a power value determined by Equation 3 immediately before being transmitted through a physical antenna.
In the LTE system, since a single power amplifier is used in uplink, only signal-antenna transmission is possible. However, since multiple antennas may be used in downlink, a problem such as imbalance in maximum or average system throughput between uplink and downlink occurs in the LTE system. Accordingly, in an LTE-Advanced (LTE-A) system, a maximum of four transmission antennas is employed in order to improve uplink system throughput. As described above, in the LTE system, since the number of power amplifiers is one, uplink power control is performed with respect to overall transmission power. However, if single power control is performed when the terminal has a plurality of power amplifiers as in the LTE-A system, efficient power control is impossible and thus a serious problem occurs in power consumption of the terminal.
Accordingly, there is a need for an adequate uplink power control method if a terminal has a plurality of transmission antennas.